1. Field of the Invention
The present invention relates to a conductive roller (e.g., a charge-imparting roller, an image-transfer roller, or a development roller) for use in an image-forming apparatus such as an electrophotographic or toner-jet-type copying machine or printer. The roller of the invention is suitable for a transfer roller serving as an intermediate transfer member or a transfer member for use in a transfer-type image-forming apparatus such as a copying machine, a printer, or a facsimile machine.
2. Background Art
Conductive rubber rollers, which have rubber elasticity and controlled conductivity, are important members in an electrophotographic process. However, the molecule of the rubber for constituting a rubber roller generally does not have resistivity required for the process (104 to 109 Ωcm), and such rubber roller employed in practice is formed from a limited rubber species such as epichlorohydrin rubber. In many cases, in order to ensure required levels of elastic modulus, mechanical strength, and temperature/moisture characteristics, electrical conductivity is imparted to a chemically stable rubber substrate such as silicone rubber, Ethylene-propylene rubber (EPDM), or polyurethane through addition thereto of conductive microparticles such as carbon black, and resistance of the roller is adjusted by forming a coating layer on the roller.
In recent years, the melting point of a toner binder for use in an electrophotographic copying machine has become lower and lower. In response to this trend, a development roller is required to have low hardness so as to provide a sufficient nip to ensure charging of the toner. Japanese Patent application Laid-Open (kokai) No. 2005-283913 discloses a development roller for satisfying the above requirement. The proposed roller, which includes an elastic layer, a urethane resin coating layer, and a thin layer formed of a hardened isocyanate, maintains softness over the entirety of the roller and has a hard surface.
Formation of a coating layer is not preferred, in that cumbersome steps are required, thereby increasing production cost. In an attempt to readily solve the problem, Japanese Patent application Laid-Open (kokai) No. 5-158341 discloses an approach including chemically treating a roller surface. Although the chemical treatment is an effective approach, a surface-treated conductive roller tends to have a hard surface. In practical use of a hardness-reduced roller, the surface of the roller is considerably deformed. Therefore, such a roller is required to have resilience in response to deformation. A roller having a hard surface-treated layer has insufficient resilience in response to deformation, and electrical resistance varies during use thereof, which is problematic.